The recent bleaching of coral reefs serves as a terrifying reminder that the threat of global warming is upon us. The increase in water temperature, along with other factors like the acidification of the oceans, has left its ghostly mark on reefs off Australia, the Maldives, Sri Lanka, Kenya, Tanzania and the Seychelles.

But why has it affected some areas and not others? A study carried out by scientists from the U.S. National Center for Atmospheric Research (Ncar) and the Australian Institute of Marine Science (Aims), reveals that it may have to do with an built-in ocean thermostat that prevents the sea surface temperature from exceeding 31 degrees Celsius. Lead author Joan Kleypas explains: “Global warming is damaging many coral but it appears to be bypassing certain reefs that support some of the greatest diversity of life on the planet.” The upshot? “In essence, reefs that are already in hot water may be more protected from warming than reefs that are not; this is rare hopeful news for these important ecosystems.”

The science goes like this: usually warm water temperatures can expel the nutrient-rich colorful microscopic organisms. This results in “bleaching” — in which the coral literally turns white. Worse still, if the water temperature doesn’t return to normal within a week or so, the bleached coral can die.

But with the ocean thermostat in place, there’s no need for concern, right? Err, wrong. The scientists warn that global warming is happening at such an alarming rate, it may overwhelm the thermostat.

“This year is the International Year of the Reef, and we need to go beyond the dire predictions for coral reefs and find ways to conserve them,” says Dr Kleypas.